In recent years, much research has been devoted to the development of fuel cell technology, particularly for automotive applications. Fuel cell power plants have shown efficiencies as high as 55%. Furthermore, fuel cell power plants are environmentally-friendly since they emit only heat and water as by-products.
Fuel cells produce energy by combining hydrogen and oxygen to produce water and an end product. In a Polymer-Electrolyte-Membrane (PEM) fuel cell, a polymer electrode membrane serves as the electrolyte between a cathode and an anode. In the PEM fuel cell, multiple fuel cells are frequently stacked in series to form a fuel cell stack. In the fuel cell stack, one side of a flow field plate serves as the anode for one fuel cell while the opposite side of the flow field plate serves as the cathode for an adjacent fuel cell. Because each flow field plate serves as both an anode and a cathode, the flow field plate is also known as a bipolar plate.
Bipolar plates for PEM fuel cells must be electrochemically stable, electrically conductive and inexpensive. The corrosion of metallic bipolar plates in the fuel cell environment accelerates the corrosion process through degradation of the membrane. The degradation products of the membrane include hydrogen fluoride (HF), which accelerates the corrosion process, causing the corrosion process to become autocatalytic in nature. 316L stainless steel has been used as an inexpensive bipolar plate material.
While 316L stainless steel exhibits a fair corrosion resistance to fluoride ions, the corrosion rate increases with the increase in the fluoride ion leach out rate. This problem can be mitigated somewhat by removing the hydrogen fluoride ions from the fuel cell environment or by using higher grades of stainless steel which are more resistant to corrosion by fluoride ions than 316L stainless steel. However, the use of higher grades of stainless steel for the bipolar plate tends to increase the cost of the bipolar plate.
Various methods are known for increasing the corrosion resistance of a corrosion-susceptible substrate. For example, US20030228512 A1 discloses a method of improving the contact resistance of the surface of a stainless steel substrate while maintaining optimum corrosion resistance of the substrate by depositing a gold coating on the substrate. US20040091768 A1 discloses a method of increasing the corrosion resistance of a substrate by providing a polymeric conductive coating on the substrate. U.S. Pat. No. 6,372,376 B1 discloses a method of increasing the corrosion resistance of a substrate by providing an electrically-conductive, corrosion-resistant polymer containing a plurality of electrically conductive, corrosion-resistant filler particles on the substrate.
It has been found that coating the surface of a lower grade stainless steel bipolar plate, such as a 316L stainless steel bipolar plate, for example, with a thin layer of high-grade stainless steel or alloy using thermal spraying imparts a high degree of fluoride ion corrosion resistance to the bipolar plate while maintaining the cost of the bipolar plate within acceptable levels. Only a small amount of the more expensive (more corrosion resistant) alloy is required.